Chapter 7: From the haploid to the functional diploid;
homokaryons, heterokaryons, dikaryons and compatibility

Most fungal mycelia contain haploid nuclei. This is a characteristic of
kingdom Fungi; unlike the other major eukaryotic groups, most true fungi are
haploid. Even in fungus-like organisms in the Oomycota (Kingdom
Straminipila) like Phytophthora infestans,
the cause of potato blight, the nuclei are diploid. This difference in ploidy is
an important contrast between ‘true’ and ‘non-true’ fungi. Of course, there are
exceptions to every rule and some true fungi are diploid, like Candida
albicans, a yeast which causes disease in
humans; and rhizomorphs and
fruit bodies of Armillaria mellea (a pathogen of trees that belongs to
the Basidiomycota)(Peabody et al., 2000).

In this Chapter compatibility and the individualistic mycelium will be our
main concerns. Formation and breakdown of heterokaryons, and the nature and
maintenance of the dikaryon are major topics, as are the mechanisms that
regulate these processes: vegetative compatibility and the incompatibility
systems. We also discuss gene segregation during the mitotic division cycle,
which culminates conceptually in what is known as the parasexual cycle. Finally,
we consider the segregations of the cytoplasmic genetic entities, mitochondria,
plasmids, viruses and prions.

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